(1) Field of the Invention
This invention relates to optical systems utilizing optical articles including focused image holograms for the storage of information, decorative applications and the like.
(2) Description of the Prior Art
The advent of holography, i.e., the technique of recording onto a photosensitive medium an interference pattern formed by a reference light beam and a second light beam coherent with the first beam and modulated by an object, has brought about a veritable revolution in optical processing. While a variety of different optical configurations are known for producing holograms, each having differing advantages and drawbacks, most prior art holographic methods suffer a common limitation of being extremely sensitive to vibration of the respective optical elements and of limited optical quality due to dust particles and the like. While techniques such as spatial filtering may minimize some of these limitations, in certain systems, such as those utilized in forming focused image holograms, such limitations are not so readily eliminated.
Nonetheless, focused image holograms are recognized as being a preferred form of interferometric recording. In U.S. Pat. No. 3,834,786 (Carlsen), a technique is disclosed for forming focused image holograms which utilizes a telecentric relay lens system in which information from an optical transparency is spatially modulated onto an object beam, passed in and out the lens system as parallel rays, thereby changing the magnification of the spatially modulated beam and thereafter coimpinged onto a photosensitive medium together with a reference beam to form the focused image hologram. In U.S. Pat. No. 3,917,378 (Gale), focused image holograms comprising superimposed parallel diffraction gratings, each modulated by a different color component, are provided as embossed patterns on a plastic medium. All such techniques suffer the same limitation in that lenses used to focus the modulated object beam onto the photosensitive medium may ultimately limit the resolution and/or spatial size of the hologram.
In an alternative technique for forming focused image holograms depicted in U.S. Pat. No. 3,820,869 (Bolusset et al), some of the disadvantages of the systems discussed above are eliminated. In the system depicted in FIG. 4 of that patent, a transparent graphic information carrying object is placed in substantial contact with a light sensitive plate and two beams of mutually coherent radiation are caused to pass through the transparent object to provide on the light sensitive plate an interference pattern which is modified by the graphic information. In that system, one of the mutually coherent beams is required to be directed normal to the plane of the light sensitive plate. The presence of optical elements such as a portion of a lens system 25 and a beam-splitting prism assembly between a spatial filter suggested to be present at a focal point of the lens system 25 and the transparency may introduce artifacts such as those produced by lens aberrations and dust. Furthermore, such a configuration limits the obtainable resolution due to shadowing effects present in a practical system in which a photographic medium having even a nominal thickness is used.